Vehicles are typically equipped with various means to enable the vehicle operator to view the rear end of the vehicle or a trailer coupled to the vehicle. There is generally a rearview mirror coupled to the upper center of the windshield providing a view directly to the rear of the vehicle. There are also typically side-view mirrors on each side of the vehicle providing views to the rear and down the side of the vehicle and any coupled trailer. When a vehicle is used to pull a trailer, the rearview mirror mounted in the center of the windshield generally cannot be used to view behind the vehicle because the view is blocked by the trailer. In this case, the driver generally must rely solely on the side-view mirrors for guidance.
Side-view mirrors on tow vehicles are often upright, rectangularly shaped devices that are manually adjusted by the driver. They are mounted far enough forward from the driver to provide a field of vision down the sides of the vehicle and coupled trailer. Some of these mirrors also have smaller convex mirrors attached that help eliminate the "blind spot" that is present between the driver's view through the side-view mirrors and the driver's view out the side of the vehicle. The field of view of the convex mirror is typically very small because they are usually mounted on the surface of the larger mirror, and distance judgement from the view is difficult due to the nature of the image on the convex mirror. The views from these manually adjusted side-view mirrors and attached convex mirrors are adequate when the vehicle and trailer are traveling straight ahead, but these views become obstructed when the vehicle makes a turn. When the vehicle turns, the path of the vehicle centerline changes with respect to the path of the trailer centerline and the view on one side will be of the side of the trailer some distance forward of the rear end. This makes it difficult for the driver of the tow vehicle to navigate a turn, particularly when the turn is made toward the passenger side of the vehicle. The driver has no easy way to maintain a line of sight with the rear end of the trailer and possibly avoid any vehicles or obstructions that may be present in the adjacent lane.
FIGS. 1A and 1B illustrate the disadvantages of a conventional vehicle-trailer mirror system. In FIG. 1A, a "tractor-type" tow vehicle 10 is coupled to a trailer 12. The tow vehicle 10 has a right side-view mirror 14R and a left side-view mirror 14L. As the tow vehicle 10 and trailer 12 change from direction D1 to direction D2 as shown in FIG. 1B, the line of sight of the right side-view mirror 14R, indicated by arrow 13, changes from being directed along the side of the tow vehicle 10 and trailer 12 to a point 16 some distance forward of the rear of the trailer 12. If another vehicle or an obstruction 18, such as a curb, is present between point 16 and the rear of the trailer 12, the driver will not be able to see it because there is no longer a line of sight between the driver and the rear of the trailer 12. There is a danger that the rear of the trailer 12 may collide with the obstruction 18 as the turn progresses since the driver may be unaware of the obstruction 18. The presence of a convex mirror would not solve the problem because the image of the obstruction 18, if present in the convex mirror's field of view at all, would be in the extreme periphery of the image, and it would be difficult for the driver to interpret the exact position of the obstruction 18 relative to the trailer 12. The line of sight is not so much of a problem when the tow vehicle 10 and trailer 12 are turning in direction D3. In this instance the driver may look out the driver side window and establish line of sight with the rear of the trailer 12. However, as illustrated by the arrow 15 in FIG. 1B, when the tow vehicle 10 is turned in the direction D2, the driver's line of sight out of the passenger window is extremely limited and does not allow the driver to see obstruction 18. It is for this reason tractor trailers frequently carry a warning on the back stating "caution, this vehicle makes wide right turns."
The lack of a means for conveniently adjusting mirrors 14L and 14R to maintain line of sight with the rear of the trailer 12 when making a right turn has been a longstanding problem. One attempt to remedy the problem is shown in Jensen et al. (U.S. Pat. No. 5,056,905). In the Jensen '905 patent, a wireless signal transmitter and receiver are located near the rear of the trailer. A continuous signal is emitted at a fixed angle from the transmitter. When a turn is initiated, the signal will eventually approach the mirror and will be reflected to the receiver. When the signal is received, a drive unit reorients the side mirror. This system requires a permanently mounted structure on the trailer. Thus, the driver cannot switch between trailers unless the system has been outfitted on all trailers used by that company. U.S. Pat. No. 5,132,851 issued to Bomar et al. and U.S. Pat. No. 5,719,713 issued to Brown show ways to use ultrasonic transducer systems to measure the variation in the distance between the back of the vehicle cab and the front of the trailer as a turn is made. Measurements from multiple sensors are used to establish the angle between the vehicle and the trailer. This system does not require modification of the trailer, but it is a complex system with significant installation problems and cost. Swedish Published Application No. 447,087 shows a similar system for reflecting a signal off the front of the trailer. Other attempts have been made by taking angular measurements at a "fifth wheel" location near the trailer hitch with proximity sensors (see U.S. Pat. No. 5,835,291 issued to Takayama) and mechanical means (U.S. Pat. No. 5,627,688 issued to Valentino and U.S. Pat. No. 5,684,647 issued to Rouleau). These inventions electronically or physically measure the relative angle between the vehicle and the trailer to adjust the side mirrors to compensate for any deviance.